Class MulticastSocket

The multicast datagram socket class is useful for sending
and receiving IP multicast packets. A MulticastSocket is
a (UDP) DatagramSocket, with additional capabilities for
joining "groups" of other multicast hosts on the internet.

A multicast group is specified by a class D IP address
and by a standard UDP port number. Class D IP addresses
are in the range 224.0.0.0 to 239.255.255.255,
inclusive. The address 224.0.0.0 is reserved and should not be used.

One would join a multicast group by first creating a MulticastSocket
with the desired port, then invoking the
joinGroup(InetAddress groupAddr)
method:

When one sends a message to a multicast group, all subscribing
recipients to that host and port receive the message (within the
time-to-live range of the packet, see below). The socket needn't
be a member of the multicast group to send messages to it.

When a socket subscribes to a multicast group/port, it receives
datagrams sent by other hosts to the group/port, as do all other
members of the group and port. A socket relinquishes membership
in a group by the leaveGroup(InetAddress addr) method.
Multiple MulticastSocket's may subscribe to a multicast group
and port concurrently, and they will all receive group datagrams.

Sends a datagram packet to the destination, with a TTL (time-
to-live) other than the default for the socket. This method
need only be used in instances where a particular TTL is desired;
otherwise it is preferable to set a TTL once on the socket, and
use that default TTL for all packets. This method does not
alter the default TTL for the socket. Its behavior may be
affected by setInterface.

If there is a security manager, this method first performs some
security checks. First, if p.getAddress().isMulticastAddress()
is true, this method calls the
security manager's checkMulticast method
with p.getAddress() and ttl as its arguments.
If the evaluation of that expression is false,
this method instead calls the security manager's
checkConnect method with arguments
p.getAddress().getHostAddress() and
p.getPort(). Each call to a security manager method
could result in a SecurityException if the operation is not allowed.

Parameters:

p - is the packet to be sent. The packet should contain
the destination multicast ip address and the data to be sent.
One does not need to be the member of the group to send
packets to a destination multicast address.

ttl - optional time to live for multicast packet.
default ttl is 1.

Throws:

IOException - is raised if an error occurs i.e
error while setting ttl.

SecurityException - if a security manager exists and its
checkMulticast or checkConnect
method doesn't allow the send.